Dexmedetomidine and haemodynamic responses to acute central hypovolaemia in conscious rabbits

1. Effects of the alpha(2)-adrenoceptor agonist dexmedetomidine on vasocons trictor and heart rate (HR) responses to acute central hypovolaemia were st udied in eight chronically instrumented rabbits. We compared intravenous (i .v.) and fourth ventricular (V-4) dexmedetomidine (0.1-10 mu g/kg) and the reversal of effects by the alpha(2)-adrenoceptor antagonist idazoxan and th e opioid agonist alfentanil. 2. Gradual inflation of an inferior vena cava (IVC) cuff reduced cardiac in dex (CI) by 8%/min, with progressive vasoconstriction and increased HR. In control rabbits, at approximately 40% baseline CI, there was sudden decompe nsation with failure of vasoconstriction and a fall in mean arterial pressu re (MAP). 3. Dexmedetomidine (i.v. and V-4) reduced resting MAP and HR and caused an earlier decompensation during central hypovolaemia. Intravenous dexmedetomi dine (3 and 10 mu g/kg) also reduced the slope of the initial vasoconstrict or response and the maximum HR. 4. The effects of dexmedetomidine were reversed by the antagonist idazoxan, which prevented the decompensation phase. Intravenous alfentanil was also effective in restoring the vasoconstrictor response and delaying decompensa tion with hypovolaemia after dexmedetomidine. Combining dexmedetomidine wit h an opioid, such as alfentanil, may provide the benefit of reduced sympath etic tone without increased risk of cardiovascular collapse.